1. Field Of The Invention
The invention relates to a catalyst containing a vanadium/phosphorus complex oxide, for the partial oxidation of hydrocarbons to maleic anhydride. The invention also relates to a process for the preparation of such catalyst. The invention further relates to a reduction treatment for improving the performance of such catalyst.
2. Prior Art
It is known to employ V-P-O complex oxides as catalysts for the preparation of maleic anhydride from n-butane. As an example, it is known from U.S. Pat. No. 3,293,268 that a V-P-O complex oxide is prepared from an aqueous solution of hydrochloride acid using V.sub.2 O.sub.5, H.sub.3 PO.sub.4 and oxalic acid as raw materials. The V-P-O complex oxide is separated by evaporating the reaction mixture to dryness. After shaping and calcination of the V-P-O complex oxide, the molar yields obtained at a temperature of 550.degree. to 575.degree. C. are only 35 to 40 percent.
U.S. Pat. No. 4,100,106 reports an improvement. The V-P-O complex oxide is precipitated from the concentrated hydrochloride aqueous solution by addition of water. This permits obtaining a crystalline product, characterized by X-rays, which after proper conditioning affords molar yields of about 55 percent at 400.degree. to 420.degree. C. when utilized as a catalyst in the oxidation of butane to maleic anhydride. This procedure also has the drawback of employing highly corrosive hydrochloric acid.
The reduction of V.sub.2 O.sub.5 with organic compounds has been well-known for some time: J. S. Littler and W. A. Waters, J. Chem. Soc., (1959), pages 1299 to 1305. Concerning the reduction of V.sub.2 O.sub.5 in alcoholic solutions containing HCl; see Koppel et al., Zeit. Anorg. Chem. 45, pages 346 to 351 (1905).
U.S. Pat. No. 3,864,280 describes the preparation of a V-P-O complex oxide in organic solvent in the presence of gaseous HCl. Once the reduction of V.sub.2 O.sub.5 is terminated, H.sub.3 PO.sub.4 is added and the V-P-O complex oxide is separated by evaporation of the organic solvent. The V-P-O complex oxide is conditioned according to a very strict procedure up to 480.degree. C. and transformed into a crystalline phase called "phase B", which represents the active catalyst. The conditioning phase of this catalyst is of utmost importance for the results of its performances and must be carried out following a procedure that is not easy to realize industrially (i.e., increase in temperature by 3.degree. C./min.). Further, this patent describes an intrinsic surface of the catalyst of from about 7 to 50 m.sup.2 /g and states that the higher the intrinsic surface area, the more active is the catalyst.
West German Patent Application OS No. 3,010,710 reports the preparation of a V-P-O complex oxide in an organic solvent: the V.sub.2 O.sub.5 reduction to vanadium IV is performed in the presence of H.sub.3 PO.sub.4. All of the reagents are added to the organic solvent at the beginning of the operation. The catalyst thusly obtained requires a very long activation: the best performances are obtained only after 10 to 20 days, with loss in production capacity.
In all methods of preparation of the V-P-O complex oxides reported up to the present time, the vanadium V compound is present in the reaction mixture from the beginning. In some cases, the phosphorus compound is added at the end of the reduction of the vanadium V to vanadium IV. In some other cases, it is present in the reaction mixture since the beginning.